Procedurally transform subquery into join

Question

Is there a generalized procedure or algorithm for transforming a SQL subquery into a join, or vice versa? That is, is there a set of typographic operations that can be appli

Answer 1

A fully automatic system for transforming queries from sub-queries into joins would be relatively difficulty to build. You would need to take an input query, parse it into a parse tree and then perform some fairly complex pattern matches on the parse tree - replacing sections of the tree with new sections of the parse tree. At the end you do a traversal of the tree to output the new query.

There can be some amazingly good or bad performance repercussions. Sometimes a sub-query is much faster than a join. Sometimes it is the inverse.